The separation of the additives, degradation products and contaminants of used oils is normally carried out by distillation methods. In the patent WO 9407798 (Viscolube Italiana Sp, 1994) a typical distillation method is described which uses high temperatures under vacuum distillation (around 350° C.) with the aim of breaking down the additives, which also deteriorates the lubricant bases, which acquire colour and odour and contain oxygenated products. To obtain quality lubricant bases by distillation at high temperature it is necessary to use final refining treatments with acid and earths or by hydrogenation, which are onerous and complicated.
Also, the high temperatures employed cause fouling of the industrial equipment, which involves stopping the process for cleaning.
As an alternative to the separation of the additives by distillation, the chemical demetallization has been developed by reacting the metal additives of the used oil with reagents that form metal salts. Thus, in the patent U.S. Pat. No. 4,247,389 (Phillips Petroleum US, 1981) the used oil is treated with solutions of ammonium phosphate at temperatures of 320-420° C. However, the demetallization treatment at these temperatures and the subsequent distillation of the demetallized oil also produce odour and colour and yield unstable products, which require treating the bases obtained by hydrogenation or by adsorbents, with the already indicated disadvantages of these final refining processes.
On the other hand, there are some references to the use of alkaline hydroxides in the regeneration of used oils. Thus, in patent DE 3433336 (BUSS AG, 1985) treatment with alkaline hydroxides is employed on the used oils before proceeding to the separation of the additives and asphalts by distillation.
Other methods use alkaline treatment of the lubricant fractions obtained after separating the additives and asphalts by distillation (US 4834868, F. J. Lappin 1989, and WO 9826031, Sotulub, Tunisia 1997); or by extraction with solvents and distillation (PCT/ES02/00354 Sener, Spain 2002), achieving in this way lubricant bases which do not require final refining treatments by acid/earths or hydrogenation.
However, no antecedents have been about methods which use treatments with alkaline hydroxides before, during or after chemical demetallization of used oils.
The present invention consists in a method of chemical demetallization followed by a distillation process of the demetallized product in the presence of alkaline hydroxides, both carried out at moderate temperatures in such conditions that lubricant bases are obtained with good characteristics of odour and colour, acidity and copper corrosion test and complying with other typical specifications of lubricant bases of first refining.